The process of revision constitutes an important aspect of the compositional technique and has been practised by composers throughout the years. This phenomenon is characteristic of any culture and it is observed in all genres of music. This study concentrates on the revision process in the twentieth century. It provides a set of categories representative of different kinds and aspects of revision that are encountered and can be verified in twentieth-century music pieces. These categories not only offer a detailed look at the technical facets of revision, but they also shed light on the reasons that lead composers to revise; revisions and reasons behind them can in some cases be interwoven.